This invention relates to a novel process for the production of a random copolymer containing repeating polyimide units and repeating polyetherimide units. More particularly, the invention is concerned with making polyetherimides from two or more dianhydrides having different reactivities.
Polyetherimides having polyimide units and polyetherimide units are disclosed in U.S. Pat. No. 3,983,093. In the production of such polyetherimide copolymers, a plurality of dianhydrides of different reactivities are reacted with one or more diamines. Generally, staged reactions of the dianhydrides and the diamines are required to obtain a random rather than a block copolymer. In practice, this is accomplished by adding a solvent, dianhydride, diamine, chain stopper, and catalyst to a reactor and heating it to 120.degree. C. or above to drive off the water from the condensation reaction. A second, more reactive dianhydride, is then added to this solution of oligomers. The reactor batch is then heated to complete the imidization reaction, after which it can be devolatized.
Unfortunately, due to the disparity in reactivities of the dianhydrides employed in making these copolymers, a number of process difficulties arise. Charging the second dianhydride at temperatures above 120.degree. C., results in a rapid evolution of water. This evolution of water causes severe foaming, which can result in loss of starting material by overflow into the distillation system. These losses can be minimized by conducting the reaction with smaller batch sizes, which unfortunately limits the productivity of this process. A second process problem is that a high viscosity solution can result at the low temperatures at which the second dianhydride is added. For example, when 2,2-bis[4(2,3-dicarboxyphenoxy)phenyl]propane dianhydride is the first dianhydride and pyromellitic dianhydride is the second, viscosities above 500,000 cps can be encountered at 120.degree. C. and when the pyromellitic dianhydride level is greater than 18 mole percent of the total dianhydrides. A third process problem is controlling the stoichiometry during the reaction. In condensation polymerizations of this type, stoichiometry is preferably controlled to within plus-or-minus 0.2%. In the staged reaction process, losses of the second dianhydride can occur as a result of entrainment of that solid reactant in the solids charging system. Additionally, water, which is generated upon addition of the dianhydride, can react with remaining dianhydride, forming the diacid or tetraacid which may not fully react with the amine end-groups of the oligomers.
Therefore, there exists a need for an efficient process for the production of a random copolymer containing repeating polyimide units and repeating polyetherimide units which overcomes the problems discussed above.